1. Field of the Invention
This invention relates to luminescent devices which typically include a laminate of a number of planar layers, one of which has a fluorescent material such as phosphor particles for directly modifying electromagnetic radiation directed thereat into luminous energy.
2. Description of the Prior Art
Image intensifying screens are frequently used to convert images contained in radiation of a given wavelength such as x-rays into another wavelength more suitable for a given use, such as visible light for fluoroscopy, and UV or like wavelengths within the spectral sensitivity range of films for radiography. Such screens typically include a backing or support layer of paper, metal or plastic which is transparent to the incoming radiation, a light reflecting coating, luminous layer composed of the finely divided phosphor particles embedded in a plastic matrix, and a light transparent plastic protective layer.
Such an image itensifying screen functions as follows: energy dissipated in phosphor particles within the luminous layer subsequent to the interaction with a source of radiation such as x-rays is converted into a number of light photons which are emitted in all directions. Most of the photons directed toward the exit face of the screen pass therethrough such that they may impinge on a light sensitive medium positioned adjacent the screen to form a latent image therein which corresponds to the spatial distribution of radiation dissipated within the luminous layer. Other photons directed toward the back face may be reflected by the reflective coating toward the exit face such that they also pass therethrough and hence add to the initially forward directed photons, thereby increasing the speed or effective efficiency of the screen. Laterally directed photons are ultimately diffused within the phosphor layers, as a result of multiple scattering and eventual absorption. Preferably, the phosphor material is provided in particulate form to minimize the lateral diffusion of the photons, and consequent loss of resolution of the converted image.
"Speed" and "resolution" are two factors which characterize the performance of such screens. Speed is a measure of the number of light photons exiting the face of the screen for a given amount of energy impinging upon it, while resolution is a measure of the ability of the screen to faithfully reproduce an image contained in the radiation source.
Structural features of the screens which contribute to speed generally conflict with those required for good resolution, such that screen constructions must compromise between speed and resolution, leaning toward one or the other depending upon the demands of intended applications. High-speed, low resolution screens have thick phosphor layers with large particles, and generally include a light reflecting coating to reflect photons toward the exit face, thereby increasing the effective speed of the screen.
In contrast, high resolution screens are generally provided with some means of inhibiting reflections from the back face of the luminous layer, such that photons emitting toward the rear face are lost, thereby resulting in a slower effective speed.
For example, in U.S. Pat. No. 3,917,950 (Carlson) there is depicted a fluorescent screen in which a carbon black light absorbing layer is provided on the back side of a fluorescent layer to improve resolution, at the expense of intensity. In U.S. Pat. No. 3,783,297 (Houston) there is depicted an x-ray intensifier device including a waffle-like support structure in which the floor of the structure is reflective and the sides are absorbant. The reflective floor is intended to reflect normally directed light, thereby intensifying the desired light, while laterally directed light is absorbed by an absorbing material, thereby minimizing the lateral spreading of light. That patent also suggests that a jagged surface on an Al face plate, such as may be formed by etching may also be employed to serve a similar function. U.S. Pat. No. 4,011,454 (Lubowski et al) depicts an alternate embodiment of a waffle-like structure included in an x-ray screen to inhibit lateral spreading of light, in which the phosphor layer itself is deposited in columns. Laterally directed light is thus reflected or contained within each column.